Drive gearing for generator of electrical signal pulses

ABSTRACT

The generator includes a permanent magnet and a concentric stationary coil, mounted for relative axial translation. The permanent magnet and the coil have rest positions in which two poles, directed radially of the magnet, are aligned with two flanges on a stationary core member which supports the coil. The rest position of the magnet is determined by its magnetic field and by the contour of the core. A cooperating cam element and a cam follower element are connected respectively to the magnet and to the coil, and follow their axial movements. One of the cam elements is rotated by the rotation of the meter, through a drive gearing including two elements constructed to provide for a substantial backlash. As the meter turns, the cam and follower cause relative axial movement between the magnet and the magnet core, away from their rest positions, being driven slowly by the meter. The backlash between the gearing elements is taken up at this time by the driving force acting through the cam and follower. Substantially no current is induced in the coil at this time. When the cam follower reaches the end of a helical riser on the cam, the follower drops off the riser of the cam and into a recess on the follower so that the magnet is restored to its rest position, with a slight amount of over-travel. During this rapid return, a sharp pulse of electrical current is produced in the coil and is delivered to appropriate pulse counting mechanism located remotely from the meter. The backlash in the gearing is freed at this time, so that there is no frictional engagement between the cam and follower during this rapid return.

m1 3,801,841 Apr. 2, 1974 DRIVE GEARING FOR GENERATOR OF ELECTRICAL SIGNAL PULSES Primary Examiner-J. D. Miller Assistant ExamirierRobert J. Hickey Attorney, Agent, or FirmCooper, Dunham, Clark, Griffin & Moran [57] ABSTRACT The generator includes a permanent magnet and a concentric stationary coil, mounted for relative axial translation. The permanent magnet and the coil have rest positions in which two poles, directed radially of the magnet, are aligned with two flanges on a stationary core member which supports the coil. The rest position of the magnet is determined by its magnetic field and by the contour of the core. A cooperating cam element and a cam follower element are connected respectively to the magnet and to the coil, and follow their axial movements. One of the cam elements is rotated by the rotation of the meter, through a drive gearing including two elements constructed to provide for a substantial backlash. As the meter turns,

the cam and follower cause relative axial movement between the magnet and the magnet core, away from.

their rest positions, being driven slowly by the meter. The backlash between the gearing elements is taken up at this time by the driving force acting through the cam and follower. Substantially no current is induced in the coil at this time. When the cam follower reaches the end of a helical riser on the cam, the follower drops off the riser of the cam and into a recess on the follower so that the magnet is restored to its rest position, with a slight amount of over-travel. During this rapid return, a sharp pulse of electrical current is produced in the coil and is delivered to appropriate pulse counting mechanism located remotely fromthe meter. The backlash in the gearing is freed at this time, so that there is no frictional engagement between the cam and'follower during this rapid return.

. a ms llptew n fig s Ill [III/[Mil]! (Jib-u:

DRIVE GEARING FOR GENERATOR OF ELECTRICAL SIGNAL PULSES CROSS-REFERENCE This invention is an improvement on the invention disclosed in US. Pat. No. 3,68l,63l, issued to Franz I-Iaener on Aug. l, 1972.

BRIEF SUMMARY OF THE INVENTION to the poles when the assemblies are in their rest position. One of the assemblies carries acylindrical cam element and the other assembly carries a cam follower element which cooperates with the cam. The cam illusra has s s is r s r r a e a d was and separated by recesses which extend below the bottoms of the risers. The follower illustrated has two projections which cooperate with the cams. At least the cam element of one assembly is rotatable, and the other cam element is fixed against rotation. In most modifications, one entire assembly rotates with its cam element.

When the assemblies are in their rest positions, the followers are axially aligned with the bottoms of the risers, so that upon relative rotation of the cam and follower elements, the followers readily engage the riser surfaces, and the movable assembly is driven axially of the generator away from its rest position. When the followers reach the ends of the helical risers and become aligned with the adjacent recesses, the movable assembly returns quickly to its rest position, generating a pulse in the coil, which is transmitted to a suitable pulse counting mechanism.

The rotatable cam element is driven through a gearing which includes two cooperating gear elements having a predetermined amount of backlash. When the cam is moving up the riser, the reactive force between the cam follower and the riser takes up the backlash in one direction. When the follower reaches the top of the riser, and starts to fall into the adjacent recess, the reactive force is relieved, and no substantial frictional force can be created between the cam and the follower. If any friction occurs between the follower face and the recess in the'cam, then the follower and its associated gear are moved forward slightly, as permitted by the backlash, so that the follower is free to move rapidly into the recess on the cam.

IN THE DRAWINGS FIG. 1 is an elevational view of a pulse generator embodying the invention, with a cover shown in crosssection.

FIG. 2 is a cross-sectional view on the line 22 of FIG. 1.

FIG. 3 is an enlarged and developed view taken along the line 33 of FIG. 2, showing the gear train in a single plane, and showing the movable magnet assembly in rest position.

FIG. 4 is a fragmentary view similar to a part of FIG. 3, but showing the magnet assembly at its maximum displacement from rest position.

FIG. 5 isa fragmentary, developed view of the cam and follower structure, with the parts in their rest positions. I

FIG. 6 is a view similar to FIG. 5, but showing the cam follower starting up the riser of the cam.

FIG. 7 is a view similar to FIGS. 5 and 6, showing the follower about to drop off the top of the riser.

FIG. 8 is a view similar to FIG. 5, showing the position of the follower during overtravel when it has just dropped off the riser.

FIG. 9 is a cross-sectional view taken on the line 99 of FIG. 3.

FIG. 10 is a perspective view, taken from below and to the right as viewed in FIG. 3, showing one of the backlash gear elements.

FIG. 11 is a perspective view, taken from above and to the right as viewedin FIG. 3, showing the other backlash gear element.

FIGS. 1-9

A pulse generator embodying the invention is shown generally at 1 and is mounted on the top plate 2 of the register of a water meter (not shown). The output shaft 3 of the meter register drives a shaft 5 through a oneway clutch 4. The shaft 5 extends above the top of the register box 2 and carries a calibration dial 6, a dial locating hub 7 and a pinion gear 8. The shaft 5 also drives a cyclometer 9 in the water meter through a suitable connection, shown as a pair of bevel gears.

The pulse generator comprises a base plate 10 and a top plate 11, held together by a pair of through bolts 12. A shaft 13 is fixed in the base plate 10 and the top plate 11 and rotatably carries a gear 14 cooperating with the pinion 8 and a pinion 15 fixed to gear 14 and cooperating with a gear 16 which is one of a pair of backlash gears 16, 17 (see FIGS. 911) rotatably mounted on a shaft 18, fixed at its ends in the base plate 10 and the top plate 11.

A permanent magnet assembly 20 is mounted for rotation and for vertical sliding motion on a shaft 21. The

assembly 20 includes a gear 22, meshing with the gear 17, and an annular permanent magnet 23 fixed on the gear 22 and surrounded by a mild steel shell 24. The permanent magnet 23 is magnetized so that it has two radially directed annularly distributed poles indicated by the letters N and S in FIG. 4. The two poles are spaced apart along the axis of the magnet assembly and are directed radially of the axis.

The permanent magnet 23 is preferably a ceramic magnet. The magnet material can either be a nonoriented or a partially oriented grade, although the latter is preferred. The mild steel shell 24 can be omitted, but is required in order to obtain the strongest magnetic field possible with a given magnet.

The permanent magnet assembly also includes a cylindrical cam follower 28, located just outside the periphery of the permanent magnet shell 24 and fixed to or preferably integral with the gear 22, as shown. The cam follower 28 includes a pair of downwardly projecting follower members 28a.

The permanent magnet assembly cooperates with a magnetic circuit assembly 25, which includes a core 26 having upper and lower flanges 26a and 26b, and a coil 27 received between the flanges. The coil 27 is .pro-

vided with appropriate electrical connections which may extend through openings in the bottom flange 26b and the base plate 10, as shown. 2

The magnetic circuit assembly 25 includes a cylindrical cam 30, which may be fixed to the base 10, as shown, and includes a pair of upwardly extending helical risers 30a separated by recesses 30b.

The magnet 23 should be strong enough so that when the two assemblies are brought together in the position shown in FIG. 3, the magnet assembly will assume a rest position where it is supported solely by the attraction between its poles and the flanges 26a and 26b, without engaging any underlying part.

Referring to FIG. 5, it may be seen that at this time the lower ends of the projections 28a of the cam follower 28 are above the bottoms of the helical risers As the meter starts to run, the magnet assembly 20 is rotated, and the cam followers 280 move toward the left in FIG. 5, engaging the helical risers 30a, as shown in FIG. 6. The permanent magnet assembly 20 is thereby driven axially of the magnetic circuit assembly 25, but the motion is so slow that any potential generated in the coil 27 is not sufficient to produce a substantial output signal. When the projections 28a pass the tops of the risers 30a, where they are shown in FIG. 7, the magnet assembly is attracted by the cooperation of the magnet poles and the flanges 26a and 26b, and moves through an output stroke from its position of maximum displacement back to the rest position. This output stroke takes place suddenly, and produces a rapid change in the magnetic flux through the coil 27, thereby producing an output pulse signal which indicates that the driving meter has turned far enough to rotate the magnet assembly 20 through one-half revolution.

When the magnetic assembly moves back to its rest position suddenly, it moves due to its own inertia beyond that rest position with a slight amount of overtravel, as shown in FIG. 8. It is desirable to allow a slight amount of overtravel, because if the magnetic assembly were to be stopped abruptly in its rest position, each output pulse would be accompanied by a substantial impact. However, it is necessary to limit the overtravel, in order tomake sure that the over-travel does not continue long enough to produce a reverse output signal in the coil 27, of sufficient magnitude to operate the pulse counting register to which the coil is connected. An overtravel equal to about percent of the stroke (i.e., the axial distance between rest position and maximum displacement) is considered to be' optimum, for a generator to be used with an electromechanical counter.

The overtravel is limited by engagement of a conical projection 22a on the underside of gear 22, with a mating conical recess 260 in the upper surface of core 26. These surfaces take the impacts, if any, at the end of the overtravel, so that there is no impact on any of the cam and follower surfaces, or on the bottom of the magnet 23. The stress due to any impact is distributed over the entire area of the mating conical surfaces.

The base plate 10, which supports the entire generator assembly, is fastened to the top plate of the meter register by means of two screws 31. A cover 32 encloses the generator assembly I on the top of the meter register.

If, for some reason, the flow of water through the meter'reverses, the shaft 3 necessarily reverses. If the one-way clutch4 does not immediately slip, then it drives the gears 8, 14, 16, 17 and 22 backward. During this backward motion, the cam follower 28 moves to the right in' FIG. 5, in the opposite direction to the arrow shown therein. This motion can continue only until the vertical surfaces of the cam follower members 28 contact the vertical faces of the helical risers 300. When this occurs, the one-way clutch must necessarily release, so that the cam follower 28 cannot be driven backward more than 180. The backward rotation of the cyclometer 9 is limited by a corresponding amount, since it is connected to and turns in unison with the cam follower 28.

Thus, there is no loss of synchronization between the meter register cyclometer 9 and the pulse counting mechanism located remotely from the meter. On resumption of flow through the meter in the normal direction, the driving member of the clutch resumes its normal driving direction, and reengages the clutch driven member, which in turn drives both the cyclometer 9 and the shaft 5 of the pulse generator in their normal directions.

FIGS. 9-11 These figures illustrate in detail the backlash gears 16, 17, which are rotatably mounted on the shaft 18.

The lower gear 16 has a single. diametrically extending tooth 16a projecting from its upper end. A cylindrical boss 16b projects upwardly from the upper flat surface of the tooth 16a. The lower end of the gear 17 is provided with a pair of projecting segments which define between them a recess 17b for receiving the tooth 16a. The tooth 16a is made slightly narrower than the recess 17b, as best seen in FIG. 9, so that the gear 17 is freely rotatable through a small angle with respect to the gear 16. This small angle of rotation provides the backlash between the two gears 16 and 17. The segments 17a are axially shorter than the combined height of tooth 16a and boss 16b, so that the only contact between the gears 16 and 17 is at the surface of the boss 16b.

When the projections 28a of cam 28 reach the upper end of the risers 30a, as shown inv FIG. 7, and drop onto the recesses 30b, the backlash between the gears 16 and 17 allows that motion to take place freely, with no substantial frictional contact between the right-hand side of the projection 280, as viewed in FIG. 7 and the left-hand side of the recess 30b. The backlash provided between the gears 16 and 17 allows the projection surfaces to separate from the recess surfaces, so that the downward movement of the permanent magnet assembly 20 takes place rapidly and generates a strong signal in the coil 27.

As the projections 280 are driven up the surfaces of the risers 300, a torsional deflection is introduced into the driving gear train, due to the interaction of the driving torque applied through the pinion l5 and the opposing torque due to the weight of the permanent magnet assembly 20 and the friction between the projections 28a and the risers 30a. When the projections reach the top of the risers 30a, the opposing torque is suddenly released, so that the torsional deflection in the gear train causes the gears 16 and 17 to separate by an angular displacement of the gear 17 no greater than the angle of backlash allowed by the spacing between the teeth 16a and the segments 17a. The recesses 30b are sufficiently wider than the projection 280 so that after the parts leave the position of FIG. 7, the gear 17 and meshing assembly 20 can turn through the full backlash angle without moving the projections 28a far enough to engage their front edges with the left-hand sides of the recesses 30b, as viewed in FIG. 7.

I claim:

1. A generator of electrical signals, comprising:

a. a permanent magnet assembly;

b. a magnetic circuit assembly;

c. means supporting the magnet assembly and the magnetic circuit assembly for relative movement in one direction;

d. means holding one of the assemblies against movement;

e. means including the magnet assembly biasing the other assembly toward a position of rest with respect to said one assembly;

f. means for driving the other assembly relatively slowly away from its rest position for a predetermined distance and then releasing it for rapid restoration by the biasing means to its rest position, said driving means including: -l. a first cam element connected to the other assembly for movement therewith in a second direction transverse to said one direction;

2. a second cam element connected to said one assembly and cooperating with the first cam element;

3. means for moving the other assembly so that the cam elements engage and produce relative movement of the two assemblies in said one direction, said moving means including cooperating gears;

4. a common shaft supporting said cooperating gears in axial alignment;

5. a diametrically extending tooth on the end of one gear adjacent the other gear;

6. said other gear having a recess in its face adjacent said diametrical tooth, said recess being substantially wider than said tooth to provide backlash;-

7. one of said cam elements including a sloping riser terminating atits upper end at a recess extending in said one direction;

8. the other cam element including a projection smaller than said recess, which engages the riser during movement of the other assembly in said second direction, and which becomes aligned with the recess upon passing beyond the high end of the riser, whereupon the biasing means becomes effective to restore the driven assembly rapidly to its rest position, and said backlash allows said restoration substantially without friction between the cam elements; and

g a coil in the magnetic field of said permanent magnet assembly and having a substantial electrical signal potential induced therein during rapid axial movement of said other assembly.

2. A generator as in claim 1, in which:

a. one of said gears is substantially elongated in the direction of said shaft; and

b. said first cam element is fixed on a shaft parallel to said common shaft; and

c. an axially short gear on said first cam element slides axially of said elongated gear during movement of said first cam. element in said one direction.

3. A generator of electrical signals, comprising:

a. a cylindrical permanent magnet assembly having two poles of opposite polarity spaced apart along the axis of the assembly and directed radially of said axis;

b. a cylindrical magnetic circuit assembly concentric with the magnet assembly and comprising:

1. two radially extending flanges spaced apart along said axis by a distance substantially equal to the axial spacing between the poles; and

2. an axially extending core connecting said flanges;

c. means supporting the magnet assembly and the magnetic circuit assembly for relative axial movement;

d. means holding one of said assemblies against axial movement;

e. means including said magnet assembly biasing the other assembly toward a rest position in which the poles are radially aligned with the flanges;

f. means for driving the other assembly relatively slowly away from its rest position for a predetermined distance and then releasing it for rapid restoration by the biasing means to its rest position, said driving means comprising:

1. a first cylindrical cam element encircling the axis and connected to said other assembly for axial movement therewith;

2. a second cam element encircling the axis and connected to said one assembly and cooperating with the first cam element;

3. means holding one of the cam elements against rotation; and

4. means for rotating the other cam element so that the cam elements engage and produce relative axial movement of the two assemblies, said rotating means including cooperating gears;

5. a' common shaft supporting said cooperating gears in axial alignment;

6. a diametrically extending tooth on the end of one gear adjacent the other gear;

7. said other gear having a recess in its face adjacent said diametrical tooth, said recess being substantially wider than said tooth to provide backlash;

8. one of said gears being substantially elongated in the direction of said shaft;

9. said first cam element being fixed on a second shaft parallel to said common shaft;

10. an axially short gear on said first cam element sliding axially of said elongated gear during movement of said first cam element in said one direction;

11. one of the cam elements including circularly spaced helical risers separated by axially extending recesses,

12. the other cam element including circularly spaced smaller than said recesses, which projections engage the helical risers during rotation of said driven assembly, and which become aligned with said recesses after passing beyond the high ends of the risers, whereupon said biasing means becomes effective to restore the driven assembly rapidly to its rest position and said cooperating gears allow said restoration substantially without friction between the cam elements; and

g. a coil encircling said axis and having a substantial electrical signal potential induced therein during rapid relative axial movement of said two assemblies.

4. A generator as in claim 3, in which:

a. said one gear has an axially projecting boss at the center of said tooth; and

b. the recess on the face of said other gear is defined by segments axially shorter than the combined height of the tooth and the boss.

5. A generator of electrical signals comprising:

a. a cylindrical permanent magnet assembly having two poles of opposite polarityspaced apart along the axis of the assembly and directed radially of said axis;

b. a cylindrical magnetic circuit assembly concentric with the magnet assembly and comprising:

1 two radially extending flanges spaced apart along V said axis by a distance substaritially equal to the axial spac'ifig'EaVv'ai'fi poles; 5d 1m 2 an axially extending core connecting said flanges;

0. means supporting the magnet assembly and the magnetic circuit assembly for relative axial movement;

d. means holding one of said assemblies against axial movement;

e. means including said magnet assembly biasing the other assembly toward a rest position in which the poles are radially aligned with the flanges;

f. means for driving the other assembly relatively slowly away from its rest position for a predetermined distance and then releasing it for rapid restoration by the biasing means to its rest position, said driving means comprising:

1. a first cylindrical cam element encircling the axis and connected to said otherassembly for axial movement therewith;

ing with the first cam element;

3. m eans holding o n e of the cam elements against rotation; and M 7 7' i 4. means for rotating the other cam element so that the cam elements engage and produce relative axial movement of the two assemblies, said rotating means including cooperating gears; beingsubstantially wider than said tooth to provide backlash;

8. one of said gears being substantially elongated in the direction of said shaft; 9. said first cam element being fixed on a second shaft parallel to said common shaft;

10. an axially short gear on said first cam element sliding axially of said elongated gear during movement of said first cam element in said one direction;

11. one of the cam elements including circularly spaced helical risers separated by axially extending recesses;

12. the other cam element including circularly spaced projections smaller than said recesses, which projections engage the helical risers during rotation of said driven assembly, and which become aligned with said recesses after passing beyond the high ends of the risers, whereupon said biasing means becomes effective to restore the driven assembly rapidly to its rest position and said cooperating gears allow said restoration substantially without friction between the cam elements; and

g. a coil encircling said axis and having asubs tantial I electrical signal potential indficed therein during rapid relative axial movement of said two assemblies.

UNITED STATES PATENT OFFICE CERTIFICATE 0F CORRECTION 7 Patent No. 3,801,8' Dated April 97 Albert E. Hall Inventor(s) It is certified that error appears in the above-identified patent and that said Letters Patent are herebycorrected as shown below:

Col. 6, line '60, after the word "spaced" insert projections Col. 7, line 13 through Col. 8, line 38 delete entire Claim 5.

On the cover sheet, after the Abstract, "5 Claims" should read A Claims Signed and sealed this 19th day of November W (SEAL) Attest:

McCOY mason JR. c. MARSHALL DANN Attesting Officer Commissioner of Patents 

1. A generator of electrical signals, comprising: a. a permanent magnet assembly; b. a magnetic circuit assembly; c. means supporting the magnet assembly and the magnetic circuit assembly for relative movement in one direction; d. means holding one of the assemblies agaiNst movement; e. means including the magnet assembly biasing the other assembly toward a position of rest with respect to said one assembly; f. means for driving the other assembly relatively slowly away from its rest position for a predetermined distance and then releasing it for rapid restoration by the biasing means to its rest position, said driving means including:
 1. a first cam element connected to the other assembly for movement therewith in a second direction transverse to said one direction;
 2. a second cam element connected to said one assembly and cooperating with the first cam element;
 3. means for moving the other assembly so that the cam elements engage and produce relative movement of the two assemblies in said one direction, said moving means including cooperating gears;
 4. a common shaft supporting said cooperating gears in axial alignment;
 5. a diametrically extending tooth on the end of one gear adjacent the other gear;
 6. said other gear having a recess in its face adjacent said diametrical tooth, said recess being substantially wider than said tooth to provide backlash;
 7. one of said cam elements including a sloping riser terminating at its upper end at a recess extending in said one direction;
 8. the other cam element including a projection smaller than said recess, which engages the riser during movement of the other assembly in said second direction, and which becomes aligned with the recess upon passing beyond the high end of the riser, whereupon the biasing means becomes effective to restore the driven assembly rapidly to its rest position, and said backlash allows said restoration substantially without friction between the cam elements; and g. a coil in the magnetic field of said permanent magnet assembly and having a substantial electrical signal potential induced therein during rapid axial movement of said other assembly.
 2. a second cam element connected to said one assembly and cooperating with the first cam element;
 2. A generator as in claim 1, in which: a. one of said gears is substantially elongated in the direction of said shaft; and b. said first cam element is fixed on a shaft parallel to said common shaft; and c. an axially short gear on said first cam element slides axially of said elongated gear during movement of said first cam element in said one direction.
 2. a second cam element encircling the axis and connected to said one assembly and cooperating with the first cam element;
 2. an axially extending core connecting said flanges; c. means supporting the magnet assembly and the magnetic circuit assembly for relative axial movement; d. means holding one of said assemblies against axial movement; e. means including said magnet assembly biasing the other assembly toward a rest position in which the poles are radially aligned with the flanges; f. means for driving the other assembly relatively slowly away from its rest position for a predetermined distance and then releasing it for rapid restoration by the biasing means to its rest position, said driving means comprising:
 3. means holding one of the cam elements against rotation; and
 3. A generator of electrical signals, comprising: a. a cylindrical permanent magnet assembly having two poles of opposite polarity spaced apart along the axis of the assembly and directed radially of said axis; b. a cylindrical magnetic circuit assembly concentric with the magnet assembly and comprising:
 3. means for moving the other assembly so that the cam elements engage and produce relative movement of the two assemblies in said one direction, said moving means including cooperating gears;
 4. a common shaft supporting said cooperating gears in axial alignment;
 4. means for rotating the other cam element so that the cam elements engage and produce relative axial movement of the two assemblies, said rotating means including cooperating gears; beingsubstantially wider than said tooth to provide backlash;
 4. means for rotating the other cam element so that the cam elements engage and produce relative axial movement of the two assemblies, said rotating means including cooperating gears;
 4. A generator as in claim 3, in which: a. said one gear has an axially projecting boss at the center of said tooth; and b. the recess on the face of said other gear is defined by segments axially shorter than the combined height of the tooth and the boss.
 5. a common shaft supportiNg said cooperating gears in axial alignment;
 5. A generator of electrical signals comprising: a. a cylindrical permanent magnet assembly having two poles of opposite polarity spaced apart along the axis of the assembly and directed radially of said axis; b. a cylindrical magnetic circuit assembly concentric with the magnet assembly and comprising: 1 two radially extending flanges spaced apart along said axis by a distance substaitially equal to the axial spacing between the poles; and 2 an axially extending core connecting said flanges; c. means supporting the magnet assembly and the magnetic circuit assembly for relative axial movement; d. means holding one of said assemblies against axial movement; e. means including said magnet assembly biasing the other assembly toward a rest position in which the poles are radially aligned with the flanges; f. means for driving the other assembly relatively slowly away from its rest position for a predetermined distance and then releasing it for rapid restoration by the biasing means to its rest position, said driving means comprising: 1 afirst cylindrical cam element encircling the axis and connected to said other assembly for axial movement therewith; 2 a second cam element encircling the axis and connected to said one assembly and cooperating with the first cam element; 3 means holding one of the cam elements against rotation; and
 5. a diametrically extending tooth on the end of one gear adjacent the other gear;
 6. said other gear having a recess in its face adjacent said diametrical tooth, said recess being substantially wider than said tooth to provide backlash;
 6. a diametrically extending tooth on the end of one gear adjacent the other gear;
 7. said other gear having a recess in its face adjacent said diametrical tooth, said recess being substantially wider than said tooth to provide backlash;
 7. one of said cam elements including a sloping riser terminating at its upper end at a recess extending in said one direction;
 8. the other cam element including a projection smaller than said recess, which engages the riser during movement of the other assembly in said second direction, and which becomes aligned with the recess upon passing beyond the high end of the riser, whereupon the biasing means becomes effective to restore the driven assembly rapidly to its rest position, and said backlash allows said restoration substantially without friction between the cam elements; and g. a coil in the magnetic field of said permanent magnet assembly and having a substantial electrical signal potential induced therein during rapid axial movement of said other assembly.
 8. one of said gears being substantially elongated in the direction of said shaft;
 8. one of said gears being substantially elongated in the direction of said shaft;
 9. said first cam element being fixed on a second shaft parallel to said common shaft;
 9. said first cam element being fixed on a second shaft parallel to said common shaft;
 10. an axially short gear on said first cam element sliding axially of said elongated gear during movement of said first cam element in said one direction;
 10. an axially short gear on said first cam element sliding axially of said elongated gear during movement of said first cam element in said one direction;
 11. one of the cam elements including circularly spaced helical risers separated by axially extending recesses,
 11. one of the cam elements including circularly spaced helical risers separated by axially extending recesses;
 12. the other cam element including circularly spaced projections smaller than said recesses, which projections engage the helical risers during rotation of said driven assembly, and which become aligned with said recesses after passing beyond the high ends of the risers, whereupon said biasing means becomes effective to restore the driven assembly rapidly to its rest position and said cooperating gears allow said restoration substantially without friction between the cam elements; and g a coil encircling said axis and having a substantial electrical signal potential induced therein during rapid relative axial movement of said two assemblies.
 12. the other cam element including circularly spaced smaller than said recesses, which projections engage the helical risers during rotation of said driven assembly, and which become aligned with said recesses after passing beyond the high ends of the risers, whereupon said biasing means becomes effective to restore the driven assembly rapidly to its rest position and said cooperating gears allow said restoration substantially without friction between the cam elements; and g. a coil encircling said axis and having a substantial electrical signal potential induced therein during rapid relative axial movement of said two assemblies. 